Issue 48

R. Maciel et alii, Frattura ed Integrità Strutturale, 48 (2019) 269-285; DOI: 10.3221/IGF-ESIS.48.28

The macrostructures observed present a significant defect called hook defect. This phenomenon is a result of the upward flow of material generated in the advancing side, which at the same is time transported by the tool pin pushing up a zone of un-welded material and curving that particular section up. This is how the hook is generated. The grain orientation in this zone, boundary between SZ and TMAZ, suggest that unlike in the HAZ, there has been an upward motion of material provoked by the pin. Also, the hook shape suggest a sideward flow in the upper region caused by the tool shoulder. The size and intensity of the hook will therefore be dependent on these two motions, Fig. 17, and this defect reduces the effective thickness of the top sheet. This defect was shown to be present in all the manufactured joints in this experiment independently of the applied vertical load, figures 14 b), 15 b) and 16 b).

Figure 17 : Schematic representation of hook defect formation mechanics.

Figure 18 : Schematic representation of cold lap defect formation mechanics.

Another visible defect is the cold lap defect, Figs. 14 c), 15c) and 16 c). This defect appears in the retreating side, and it is a consequence of the initial upward flow under shearing effect of the pin followed by a downward flow in order to fill the space at the bottom of the pin, Fig. 18. In all the welds manufactured this defect was quite significant and that is a result not only of the pin shearing effect but mainly due to the relatively high welding speed, 20 cm/min, which increases the pin cavity volume per tool rotation, which stimulates the downward flow to fill that gap. Both defects mentioned above result in thinning of the SLJ joints and degradation of mechanical performance because they result in stress concentration areas. However, as it may be observed in Figs. 14, 15 and 16, with an increase in vertical load applied by the tool (Fig. 14 - 400 kgf, Fig. 15 - 425 kgf, Fig. 16 - 450 kgf) there is a dampening of the defects, becoming less severe. This reduction of the defect size may be due to the higher forging force, that while maintaining welding speed and the rotational speed, constant, results in higher heat generation, which ends up allowing a better mixing of the materials. This causes the reduction of stress concentration that occurs at the tip of the hook, making the cold lap more favorable to crack generation. Therefore, the steering process was more efficient in case of Fig. 16 and suggests that the higher the load applied in the welding process the higher the joint strength will be. The adhesive layer is shown to be continuous in the cross section stopping at TMAZ, at both the advancing and retreating side. As shown in Maroni [22], this adhesive interlayer expected to increase the strength and ductility of the joint. Nevertheless, it was verified that despite the application of 0.2 mm calibrated metal strips to guarantee a uniform distribution of adhesive, there is a slight variation along the cross section which might induce some discrepancies in the laboratory. Microhardness analysis The tests were performed on hybrid joint samples manufactured with 400 kgf and 450 kgf, "Hyb_400-1" and "Hyb_450 1". The results are presented in Figs. 19 and 20. In at welding region, Fig. 19, softening is observed around the weld nugget (WN) as reported in literature. In [8] it is suggested this softening is caused by coarsening and dissolution of strengthening precipitates due to the thermal cycles the welds are subjected in the welding process. The lowest hardness value is located away from the center line, in the WN limits, about 5 mm for the upper measurements, in red, and 2.5 mm for the lower ones, the blue and the green, which is equivalent to the pin limits. In this region of minimum hardness only low density rod-shaped precipitates are present [8] which provokes the hardness to be reduced. In tensile tests this minimum hardness zone is where the fracture is located. The

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